1. Field of the Invention
The invention relates to a data transmission method, a base station and a transmitter in a telecommunication system.
2. Description of the Related Art
Power amplifiers are required in radio telecommunication systems to amplify signals before transmitting the signal, because a radio signal attenuates on the radio path. Unfortunately, high-power radio-frequency amplifiers tend to be non-linear devices and therefore they in many cases cause distortion. This distortion is expressed, for example, as Inter-Symbol-Interference or out-off-band power in adjacent frequency bands. ACLR (Adjacent Carrier Leakage Ratio) quantifies the out-off-band transmitted power and thus it must remain within specified limits.
In the prior art, there are several different methods to compensate for non-linearity of power amplifiers. Compensation of power amplifier non-linearities can be classified into three main categories: feedback, feed-forward and pre-distortion. Pre-distortion is often called pre-emphasizing.
The feedback technique is especially used in audio amplifiers. However, feedback control on radio-frequencies becomes difficult because of realtime feedback circuit implementation. A linearized multicarrier power amplifier has also been proposed. It utilizes Cartesian feedback. This technique is not applicable to wide-band transmitters, due to its rather narrow frequency band.
In the case of linearization of wide-band applications, such as WCDMA (Wideband Code Division Multiple Access), feed-forward is most commonly used in the prior art. In a feed-forward amplifier, the distortion or error signal produced in the amplifier is detected by comparing the input and output signals. The detected error signals are fed into a linear sub-amplifier to amplify them to the same level as that of the power amplifier. The amplified error signal is then subtracted from the output of the power amplifier. The problem is that the linearity of the sub-amplifier must be high and this can decrease the overall power efficiency.
In a pre-distorter amplifier, a pre-distorter adds a pre-distorting signal to an input signal in advance to cancel the distortion generated in amplifiers. The problem is that the compensating performance deteriorates if the amplifier parameters deviate from the designed values.
There are prior-art solutions for adapting a pre-distorter amplifier. The problem in the prior-art solutions is, however, that the linearization of distorted signals is inadequate. Typically pre-distortion parameters are stored in a look-up table, but this approach easily enlarges the memory size and creates quantization noise, because parameter values typically differ from each other by predetermined steps.